Rescuers have worked through the night to reach dozens of people believed to be trapped under a collapsed flyover in the Indian city of Kolkata (Calcutta).

The bypass was under construction in the Girish Park area when it collapsed suddenly on Thursday night.

At least 23 people were killed and scores were injured.

The 2km-long (1.2 mile) flyover had been under construction since 2009 and missed several deadlines for completion.

Officials said more than 90 people have been rescued, some of whom where in hospital in a critical condition.

"Many of the people rescued have been seriously injured," police chief Ajay Tyagi told the Reuters news agency.

"Many could still be buried below the debris."

The BBC's Justin Rowlatt in Kolkata says much of the rubble has been cleared.

Rescue workers have struggled to get cranes and other machinery through the narrow and congested streets of Burrabazar area where the incident happened.

An injured construction worker said he had been working on the structure before it collapsed and added that he had seen bolts sticking out of the metal girders.

Safety issues

"We were cementing two iron girders for the pillars, but they couldn't take the weight of the cement," Milan Sheikh told the AFP news agency.

"The bolts started coming out this morning and then the flyover came crashing down."

The cause of the disaster was not immediately clear, but safety issues such as lack of inspections and the use of substandard materials have plagued construction projects in India.

The company in charge of the construction, IVRCL, said it would co-operate with investigators. However one of its senior officials said in a news conference that the collapse had been "an act of god" as the company had a good safety record.

India's collapsing building problem

The flyover was in one of Kolkata's most densely populated neighbourhoods, with narrow lanes, and shops and houses built close together, making it difficult to get heavy equipment to the scene.

The BBC's Rahul Tandon in Kolkata described chaotic scenes on Thursday night, with officials trying to clear the area in case the structure collapsed further.

CCTV footage posted on social media appears to show the moment a 100-metre section of the structure collapses, hitting passers-by, auto rickshaws and nearby buildings.

Witnesses said other cars, buses and lorries were also hit. People are said to have been living in makeshift homes under the flyover.

A spokesman for the National Disaster Management Authority (NDRF), Anurag Gupta, told AFP that soldiers and NDRF personnel were at the scene alongside hundreds of police and local officials.

The chief minister of West Bengal, Mamata Banerjee, has said the authorities will take "stringent action" against those responsible for the disaster.

Members of the National Disaster Response Force search for survivors trapped under the rubble [Rupak De Chowdhuri/Reuters]

Emergency workers in Kolkata continued to search for survivors underneath a collapsed overpass in a congested area of the eastern city, as the construction firm building the flyover was being investigated.

At least 23 people were killed and over a dozens were still trapped under the rubble of the under-construction overpass that collapsed on Thursday.

Rescuers were using sniffer dogs and special cameras to find trapped people and used saws, small cranes, and their bare hands to dig through the wreckage in search of victims.

Smashed yellow taxis, destroyed rickshaws and the bloody legs of trapped people jutted from the collapsed girders and concrete slabs.

"The condition is pathetic. At this moment no one has any clue how many people are trapped," said Raichand Mohta, a police officer at the scene.

Al Jazeera's Divya Gopalan, reporting from Kolkata, said the accident occurred in a "very busy area and the rescue operation is too slow.

"Heavy machinery, cranes, diggers are trying to break through the large slab of the flyover that has fallen on to the ground to rescue people trapped under the rubble.

"However, what is making this rescue operation more difficult is that this area is very congested and densely populated. And the flyover is built in between residential buildings. 'Body part strewn all over'

Local TV footage showed a street scene with two auto rickshaws and a crowd of people suddenly obliterated by a mass of falling concrete that narrowly missed cars crawling in the traffic jam.

"Before the ambulances arrived, we were here at the location trying to rescue the people out of the rubble. There were body parts strewn all over the place," a witness told Al Jazeera.

Police said 39 of the more than 70 people rushed to hospitals were still being treated on Friday morning.

Getting survivors to hospital was also complicated by the lack of access for ambulances to the flyover. Safety standards were lax, witnesses said.

"Every night, hundreds of labourers would build the flyover and they would cook and sleep near the site by day," said Ravindra Kumar Gupta, a grocer, who pulled out six bodies, together with his friends. Investigation launched

Police launched an investigation into the construction firm building the flyover, the local government said, and had reportedly sealed its offices.

Indian company IVRCL was building the 2km Vivekananda Road flyover, according to its website, and its director of operations, AGK Murthy, said the company was unsure of the cause of the disaster.

"We did not use any inferior-quality material and we will cooperate with the investigators," Murthy told reporters in Hyderabad, where the firm is based. "We are in a state of shock."

Construction collapses are common in India, where regulations aren't strictly enforced and builders often use substandard materials.